The disposable absorbent articles such as disposable diapers, incontinence diapers, training pants and sanitary towels, are basically formed by an upper layer which allows for the passage of the liquid, a bottom impermeable layer and an absorbent core placed therebetween. The absorbent core is responsible for absorbing and retaining the liquid or semi-liquid exudates which permeate the upper layer of the article.
An absorbent core, besides complying with its primary objectives mentioned above, must be soft and flexible, so as to fit the user's body. In recent years, the market has tended to thin absorbent cores, in which, for achieving the suitable absorption and retention, superabsorbent materials have been used which may be mixed with the fibers of which the absorbent core is comprised by, or located on specific zones of the core. One of the most commonly used superabsorbent materials (SAP) in disposable absorbent articles are the sodium or potassium polyacrylate particles, yet other several synthetic or natural materials are known in the field which may be used as SAP in absorbent cores.
There are plenty of thin cores proposals that better conform to the baby's body and meet the needs of fluids absorption and retention. For example, U.S. Pat. No. 5,149,335 to Kellenberger, refers to a thin disposable diaper containing superabsorbent material within the core, ranging from 60 to 100% of SAP contained in a fibrous core. On the other hand, U.S. Pat. No. 5,098,423 to Heinz A. Pieniak, covers a thin diaper with such an arrangement that is very convenient for the user, since it is very narrow in the crotch portion and the user must not remain with his legs apart; the absorbent core of this diaper contains from 200 to 1500% of a superabsorbent hydrogel.
Mexican Patent No. 249,142 to Corona Carlos, covers an ultra thin absorbent core containing no absorbent material fibers, same only comprises a superabsorbent material located within a covering layer of a nonwoven material, which upper distributes the liquid before it permeates and is absorbed and retained by the superabsorbent material.
Furthermore, in order to streamline the operation of absorbent cores, there have been several developments incorporating zones with different specific weights and densities within an absorbent core, such is the case of the absorbent core described in U.S. Pat. No. 4,834,735 to Alemany, which covers an absorbent core with an acquisition zone and a storage zone which have different densities; the acquisition zone which is located in the front half of the core has a lower density than the rest of the core (storage zone), thus the liquid will be quickly collected by the acquisition zone and distributed and stored therein. However the liquid will very easily reach the side edges of the core as the high-density zone continues until the edges thereof; U.S. Pat. No. 5,849,002 to Corona Carlos, refers to an absorbent core comprised by 3 zones, fluid receiving zone, a primary storage zone and an anti-leakage zone, each zone has different densities and specific weights. Mexican patent MX 219,792, also to Corona Carlos, covers an absorbent core incorporating transition zones between the receiving and anti-leakage zones, in these transition zones, the density decreases gradually to avoid disruption of the core.
Nevertheless, there is still a need to develop thin absorbent cores that are soft and comfortable for the user as well as more efficient, which improve the absorption rate and liquid distribution characteristics, in this proposal, said efficiency is based in the combination of an absorbent core with layers and zones with different specific weights and densities. The core has a first upper absorbent material layer, preferably defibrated cellulose, a second absorbent material layer, preferably defibrated cellulose at the bottom of the core, and a third layer comprising a mixture of absorbent material fibers and superabsorbent material (SAP) particles, located between the first and second layers; moreover, the intermediate layer (third layer) has therein distribution-storage, transition and anti-leakage zones. Thus a thin, soft, flexible core is obtained with very good absorption rate and liquid distribution characteristics, further reducing the consumption of cellulose, and optimizing the use of the superabsorbent powder (SAP).